Semiconductor diode lasers provide an intense and efficient source of laser radiation. Continuous wave (cw) output powers in excess of one Watt have been achieved with semiconductor diode lasers by using either a broad-area or an array geometry. Such devices, however, operate in multi-lateral modes that give rise to a twin-lobed far-field emission beam in a direction parallel to an active layer of the laser diode. This twin-lobed output beam limits the ability of these conventional laser sources to be focused tightly or to be propagated over long distances. Many potential applications of high-power diode lasers, including optical radar, satellite communication and laser printers, require a single on-axis near-diffraction-limited output beam.
Although considerable effort has been expended to achieve fundamental mode operation in broad-area lasers and diode laser arrays, to date, this goal has only been realized under short-pulse (&lt;500 ns), and low-duty-cycle (.ltoreq.10.sup.-3) conditions under which junction-heating effects, which occur from the flow of current (carriers) through the material, are unimportant. However, it has beer established that junction heating plays an important role in the selection of the lasing modes in both array and broad-area devices for long-pulse and cw operation. Junction heating disadvantageously adds a positive contribution, approximately 4x10-4c-1, to the refractive index in the active region thereby producing an overall index guide that favors multi-lateral mode operation.
It is therefore an object of the invention to provide a semiconductor diode laser having a single on-axis near-diffraction-limited output beam.
It is another object of the invention to provide a semiconductor diode laser having integral structure for controllably varying a lateral refractive index profile of the diode to substantially compensate for the negative effect of junction heating during the operation of the laser diode and to thereby provide for a single on-axis near-diffraction-limited output beam.